Maximal Ratio Combining (MRC) is a technique used to combine the different multipath components from the fingers of a Rake Receiver in CDMA systems. In ideal conditions, MRC is known to provide better Signal to Interference Ratio (SIR) than Equal Gain Combining (EGC) which is another well understood combining technique.
However, MRC requires estimation of the channel impulse response. When the channel estimates are not reliable, as could happen in severe channel conditions, the performance of MRC degrades. With non-ideal channel estimates the performance of MRC can degrade below that of EGC. This is because EGC does not use any channel estimates for combining the paths and therefore its performance is independent of the channel conditions.
Channel Estimation is done on pilot symbols. In the downlink of W-CDMA, the only source of interference is the multipath interference. The pilot symbols are degraded by multipath interference. The level of degradation will depend on the level of multipath interference. Hence the reliability of the channel estimates too will depend on the level of multipath interference. Therefore for more reliable channel estimates to be obtained, the effect of multipath interference on the pilot symbols has to be removed.
It follows that a better receiver would be the one, which chooses its combining technique based on the channel conditions. The problem of the performance degradation of MRC due to non-ideal channel estimates has not been addressed so far.